1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus which reproduces information recorded on an optical disc, and more specifically, to an optical disc reproducing apparatus with the function of detecting the direction of rotation of the optical disc.
2. Description of the Background Art
In an optical disc reproducing apparatus with the function of detecting the direction of rotation of the disc, a spindle motor for driving the rotation of the optical disc is generally controlled by using a motor control signal generated based on a sync signal (frame sync) contained in a reproducing signal outputted from a light pick-up at the time of reproducing the optical disc. To be more specific, the spindle motor is controlled by a servo loop of a PLL (Phase Locked Loop) or the like, and the voltage according to the phase difference between the sync signal and the predetermined reference signal is used as a motor control signal for the spindle motor so as to control the rotation of the spindle motor.
In order to reproduce the information recorded on the optical disc, to begin with, a predetermined voltage is applied to the spindle motor so as to start the rotation of the spindle motor and the optical disc, which is followed by a phase comparison between the sync signal contained in the reproducing signal from the light pickup and a predetermined reference signal. In accordance with the results of the phase comparison, the spindle motor is accelerated until the number of rotation within a predetermined range is reached, and when the number of rotation is reached, the servo system with the PLL is employed. If the shift to the servo system with the PLL is unsuccessful for some reason after accelerating the rotation of the spindle motor, it causes a runaway phenomenon of the optical disc due to the abnormally high rate of rotation of the spindle motor, thereby making the rotation of the motor out of control.
For example, when an optical disc is shifted from one side to the other side for reproduction, the rotation of the spindle motor is accelerated until the light pickup moves to the position for reproduction on the optical disc. Consequently, the longer it takes for the light pickup to reach the proper position for reproduction, the higher rate of rotation of the spindle motor has until it finally exceeds the regulated rate of rotation, resulting in the occurrence of a runaway phenomenon of the optical disc.
Furthermore, for example, when the light from the light pickup is emitted on a mirror surface (region) having no track, which is out of the effective recording region of the optical disc, it becomes impossible to detect the sync signal and to employ the servo system with the PLL. This would sometimes cause the spindle motor to be accelerated until the optical disc runs away.
An apparatus with an FG (Frequency Generator) sensor for detecting the runaway of an optical disc is available. The configuration of an FG sensor 30 is shown in FIG. 7A. Generally, FG sensor 30 is provided to detect whether the spindle motor is rotating or not. FG sensor 30 is placed under the turntable and is composed of a combination of a reflective sheet 31 including light-reflective surfaces (silver parts) 32 and absorbance surfaces (black parts) 33 arranged alternately and a photo sensor 34.
In FG sensor 30, reflective sheet 31 rotates in the direction of an arrow 35 in synchronization with the rotation of the unillustrated spindle motor. Consequently, the repetitive pattern of the presence and absence of the reflective light in reflective surfaces 32 and absorbance surfaces 33 is detected as an output pulse signal 36 of a photo sensor 34 which receives the reflective light and in turn outputs an electric signal, and is supplied to a micro computer 37 of the optical disc reproducing apparatus. Micro computer 37 detects based on pulse signal 36 shown in FIG. 7B which is inputted from FG sensor 30 whether the spindle motor is rotating or not, and also can detect based on the cycle of pulse signal 36 whether the optical disc is running away or not due to the abnormal acceleration of the spindle motor. In the case of the runaway of the optical disc, micro computer 37 determines whether the overrunning optical disc has been stopped or not, based on the input of pulse signal 36, without applying the spindle motor with voltage. Specially providing FG sensor 30 for the detection of rotation of the disc would reverse the miniaturization of the apparatus.
Prior art techniques for controlling the runaway phenomenon of the optical disc without using FG sensor 30 include the optical disc player shown in Japanese Utility Model Laying-Open No. 7-32740; the device for controlling a DC motor shown in Japanese Patent Laying-Open No. 2000-125588; and the apparatus for detecting the rate of rotation of a DC spindle motor shown in Japanese Patent Laying-Open No. 2001-78483.
First, the optical disc player shown in Japanese Utility Model Laying-Open No. 7-32740 controls the runaway of the spindle motor so as to prevent the optical disc from running away by including: a runaway detection unit for detecting the runaway of the spindle motor which drives the rotation of the optical disc; a rotational direction detection unit for detecting the direction of rotation of the spindle motor at the time of the runaway of the spindle motor; and a brake signal supply unit for applying the spindle motor with a brake signal of the polarity according to the output of the rotational direction detection unit.
However, the runaway detection unit detects the runaway of the spindle motor by detecting the current from the spindle motor, and the rotational direction detection unit detects the direction of rotation of the spindle motor by the output voltage of the runaway detection unit. In other words, the runaway detection unit performs detection by using current, and the rotational direction detection unit performs detection by using voltage. Since current and voltage are likely to change with the operational condition, the detecting operation is not necessarily carried out in a stable manner.
The device for controlling a DC motor shown in Japanese Patent Laying-Open No. 2000-125588 controls the runaway of the DC motor so as to prevent the optical disc from running away by including: an intermittent braking voltage supply unit which intermittently supplies the DC motor with a braking voltage of the polarity able to electrically brake the DC motor (corresponding to a spindle motor); a rotational direction determination unit which reverts the rotational direction of the DC motor by detecting the polarity of the back electromotive force of the DC motor while the DC motor is not supplied with a braking voltage; and a controller which determines whether the direction of rotation of the DC motor has been reverted while the stopping of the DC motor is under control, and suspends the supply of the intermittent braking voltage to the DC motor.
The rotational direction determination unit detects the polarity of the back electromotive force of the DC motor; however, the voltage level of the back electromotive force is fairly low and unstable, making it difficult to obtain a stable determining operation.
The apparatus for detecting the rate of rotation of a DC spindle motor shown in Japanese Patent Laying-Open No. 2001-78483 controls the runaway of the spindle motor so as to prevent the optical disc from running away by including: a current-to-voltage converter which detects and converts a current supplied to the spindle motor into a voltage; an analog-to-digital converter which converts the analog voltage outputted from the current-to-voltage converter into a digital signal; and a controller which detects the rate of rotation of the spindle motor using the digital signal outputted from the analog-to-digital converter and controls the spindle motor in accordance with the detected rate of rotation.
In this technique, the direction of rotation of the spindle motor is detected by using the voltage which is based on the current applied to the spindle motor. However, the current is likely to change under load conditions, so it is difficult to keep stabilizing the operation of detecting the direction of rotation of the optical disc that is running away.
Besides the techniques mentioned above, it is also known to bring the overrunning optical disc to the stop mode by making a timer count the time from the start of the runaway without applying voltage on the spindle motor for a predetermined period of time and by determining that the optical disc has been stopped after the predetermined period of time. In this art, it takes too much time to bring the optical disc which has started to run away to the stop mode. As another problem, if the brake signal in the normal direction of rotation is applied to the spindle motor while the optical disc is rotating in the reverse direction, the optical disc is accelerated in the reverse direction of rotation and may be unable to be stopped.